Russia Ukraine war, page-105102

The Ukrainian Army has not defeated the Russians in a set piece battle since the start of Russia’s Special Military Operation.

well I'm not sure how you define a "set piece battle " IMO it's because there have been none - none of those huge tank battles of WW2 that I think you have in mind .
Techniques of war have moved on since then.

There was the battle for Kyiv where russia was forced to withdraw .
Same in Kherson - a series of small battles making russias position in the city untennable .
And then the thunder run through Kharkiv bursting through multiple russian defence lines .

Mariupol which was russias big win was not really a set battle but a creeping advance backed by huge amount of artillery .